Cell division plays a crucial role during all phases of plant development. The continuation of organogenesis and growth responses to a changing environment require precise spatial, temporal, and developmental regulation of cell division.
The basic mechanisms controlling the progression through the cell cycle appear to be conserved in all higher eukaryotes, although the temporal and spatial control of cell division can differ largely between organisms. Plants have unique developmental features which are not found in either animals or fungi. First, due to the presence of a rigid cell wall, plant cells cannot move and consequently organogenesis is dependent on cell division and cell expansion at the site of formation of new organs. Secondly, cell divisions are confined to specialized regions, called meristems. These meristems continuously produce new cells which, as they move away from the meristem, become differentiated. The meristem identity itself can change from a vegetative to a reproductive phase, resulting in the formation of flowers. Thirdly, plant development is largely post-embryonic. During embryogenesis, the main developmental event is the establishment of the root-shoot axis. Most plant growth occurs after germination, by iterative development at the meristems. Lastly, as a consequence of the sessile life of plants, development and cell division are, to a large extent, influenced by environmental factors such as light, gravity, wounding, nutrients, and stress conditions. All these features are reflected in a plant-specific regulation of the factors controlling cell division.
The unparalleled potential of plants for continuous organogenesis and plastic growth also relies on the competent or active state of the cell division apparatus. The discovery of a common mechanism underlying the regulation of the cell cycle in yeasts and animals has led to efforts to extend these findings to the plant kingdom and is leading to research aimed at converting the gathered knowledge into useful traits introduced in transgenic plants.
When eukaryotic cells and, thus, also plant cells divide they go through a highly ordered sequence of events collectively termed as the “cell cycle.” Briefly, DNA replication or synthesis (S) and mitotic segregation of the chromosomes (M) occur with intervening gap phases (G1 and G2) and the phases follow the sequence G1-S-G2-M. Cell division is completed after cytokinesis, the last step of the M-phase. Cells that have exited the cell cycle and have become quiescent are said to be in the G0 phase. Cells at the G0 stage can be stimulated to reenter the cell cycle at the G1 phase. The transition between the different phases of the cell cycle are basically driven by the sequential activation/inactivation of a kinase (called “cyclin-dependent kinase”, “CDC” or “CDK”) by different agonists.
Proteins called cyclins are required for kinase activation. Cyclins are also important for targeting the kinase activity to a given subset of substrate(s). Other factors regulating CDK activity include CDK inhibitors (CKIs or ICKs, KIPs, CIPs, INKs), CDK activating kinase (CAK) and CDK phosphatase (CDC25) (Mironov et al. (1999) Plant Cell 11, 509-522 and Won K. et al. (1996) EMBO J. 15, 4182-4193).